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Constructed wetlands are currently used as an alternative to conventional waste-water treatment systems throughout the world. However, several of the chemical processes involved are still in the process of being understood. The removal of ammonium from waste-water is one of these processes. Two mechanisms by which constructed wetlands remove ammonium from the system is by ammonium adsorption or through ammonium volatilization. The focus of this study was to discover the reasons behind abnormally high adsorption rates recorded in a previous study conducted in 2005 for a gravel currently used in one of Montana State University’s constructed wetland projects. This sorption study consisted of five experiments. The first experiment was meant to determine the Cation Exchange Capacity (CEC) of the gravel to further define the ability of the gravel to sorb ammonium in a constructed wetland system. The second addresses the basic physical properties that are involved in determining whether the amount of ammonium adsorption occurring in the gravel. The third and fourth experiments sought to establish which of the two main processes involved in the removal of ammonium was the reason behind the overall loss of ammonium from this particular constructed wetland system, and complete a mass balance of a specific amount of ammonium through an adsorption and desorption experiment. These two experiments also looked at the ability of gravel to sorb variable initial concentrations of ammonium over a period of 24 hours. The fifth experiment was meant to determine the maximum sorptive capacity of the gravel of interest. Results of this analysis showed that the gravel’s CEC was below the detectable limit of the methodology that was being used, which was calibrated for a soil. An X-ray Powder Diffraction machine was used to determine the components of the gravel, which were revealed to be mainly a type of quartz and a silica based compound as well as 10 to 20% calcium mica. Mica increased the CEC of the gravel dramatically, thus increasing the adsorption ability of the gravel. Adsorption was found to be the single cause of ammonium loss from the system and a concentration of around 500 mg/L of NH4Cl as N was discovered to saturate the system allowing for all of the adsorption sites to be filled. The pH of the solution never rose above 6.5, effectively preventing chemical reactions from converting ammonium to ammonia and disallowing volatilization to begin. The maximum a sorptive capacity was determined to be approximately 180 mg/L of NH4 as N, which is about 232 mg/L of NH4. These results are consistent with the amount of adsorption calculated in the previous research. Overall, sorption may still be affected by factors acting on the system; research opportunities to discover the extent that these factors may change the absorbance of ammonium are many.